1. Field of the Invention
This invention relates generally to power supplies and, more specifically, the present invention relates to a switched mode power supply.
2. Background Information
A form of power supply that is highly efficient and at the same time provides good output regulation to supply power to electronic devices is the switched-mode power supply. In many electronic device applications, an approximately constant voltage output characteristic is required. In order to achieve this goal, a regulator circuit with a high PWM (Pulse Width Modulation) gain is desirable.
The method of generating an approximately constant voltage output characteristic involves adjusting the duty cycle as a function of control (or feedback) current the regulator circuit receives. The slope of the duty cycle as a function of the control current is the PWM gain. A shunt regulator current is the control current in excess of the internal consumption of the chip. The regulator circuit maintains an approximately constant power supply output voltage by modulating the duty cycle based on the shunt regulator current in excess of a threshold. The pulse width can be modulated in either voltage mode or current mode. One known regulator circuit implements a voltage-mode control loop by driving a power switch with a duty cycle inversely proportional to the shunt regulator current in excess of the threshold which generates a voltage level. This voltage level is set by the current through a resistor. The current in this resistor is substantially zero until a shunt regulator current threshold is reached. The resistor current is only responsive to shunt regulator currents greater than the shunt regulator current threshold.
The voltage level signal is filtered by an RC network to reduce the effect of switching noise. This filtered voltage level signal is compared with an internal oscillator saw-tooth waveform to generate the duty cycle waveform. As the shunt regulator current increases, the duty cycle decreases. A clock signal from the oscillator sets a latch, which turns on the power switch. The pulse width modulator resets the latch, turning off the power switch. Therefore, the pulse width modulator gain is the slope of the voltage level across the resistor as a function of the current across the resistor. Thus, the resistor value is the PWM gain. In order to increase the gain (to increase the slope of duty cycle as a function of control current), it is simply required that the value of the resistor be increased. However, by simply increasing the value of the resistance the circuitry will become more noise sensitive.